Basically any Linux would be fine (Ubuntu is rather specific, since it hides a lot under the "user-friendlyness" coat).
Currently I run Gentoo as my main workstation but have been using OpenSuSE, Fedora, Debian, CentOS and Arch for hardware/software design and they all did fine (much better then Windows).
I'd have to disagree... I've been running Xilinx ISE design software (current versions) on Debian stable, and even though that OS is not supported by Xilinx, ISE runs fine on it. Debian stable is rock solid, it basically runs forever until the hardware fails. :-D But I have seen ISE crash a few times, and perhaps those crashes wouldn't have occurred on one of the supported OS'es. Also the 1st answer you get on any Xilinx forum support question, is "that OS is not supported". Usually then other posters will follow with tips & solutions, but it's still a bit annoying.
So: the vendor declares specific OS'es as supported, the more you deviate from that, the more you are 'asking' for trouble. If just getting the job done is important, I'd go with a vendor-supported OS. Or a free derivate like CentOS in the case of RHEL. Whether that's an OS you
like is then less important than maximizing the chance that the design software will work.
Original poster was asking to setup a
workstation to run a few specific vendor tools. Gentoo might work fine for you (like Debian does for me), but neither is good advice for the topic starter, if it's known that the software to run states different OS'es as supported (disclaimer: I wouldn't know which those are for the tools mentioned, but other posters already covered that).
Modern Linux distro's may look at lot alike under the hood (or not... ;-) ), but electronic design software can be very complex software suites, sometimes very picky about what it runs on. Even if it appears to work well, there might be some lesser used functions/tools that may act up. Which you'll only find out later,
after having gone through the trouble of setting everything up. Even worse is the possibility where everything seems to work, but fails in more subtle ways (like files produced which don't match your design / not correct in some way).